In many systems of this nature, one or more persons managing the system attempt to maintain a balance between cost and customer inconvenience by adjusting, by judgment in response to observed conditions, the number of servers available at the last stage of service. In the retail store, for example, some store personnel may divide their time between serving as cashiers and some other duties. When queues build up at the cashier stations, managers summon some of these store personnel to suspend other tasks and serve as cashiers. When offered customer traffic is light, managers reassign some store personnel to close down their cashier stations and perform other tasks.
An example of such a system is a retail store, wherein the first stage (or stages) of service consists of shopping, and the subsequent (last) stage of service consists of checkout and payment at a cashier's station.
Another example is a computer system or network with ample computer processing capacity but limited resources to output results of computation, so that queuing and delays occur for jobs awaiting output processing.
Management systems dependent on observation and judgment often result in having the number of servers and the resulting waiting lines oscillate between under-utilization of servers and full utilization of servers while customer queues lengthen. That system behavior results primarily because some lead time is required from when a decision is made to reassign a server and the actual change in what the person or resource is doing. In addition, in many systems, delays in the manager's recognition of the changing situation contribute further to keeping the system out of synchrony with the customer traffic.